1. Field of the Invention
The present invention relates generally to a compressor for use in an automotive air conditioning system or the like and more specifically to a variable displacement wobble plate type compressor for such a system.
2. Description of the Prior Art
FIG. 1 schematically shows an air conditioning system of the type to which the present invention is applied. This system includes a compressor 1 which is driven by a prime mover (not shown) such as an internal combustion engine of an automotive vehicle via a belt and pulley arrangement. In this instance the pulley 2 is associated with a magnetic clutch via which the drive connection between the engine and the compressor can be selectively established. The clutch in this instance is depicted schematically as element 2a.
The discharge port of the compressor is fluidly connected with an air cooled condensor 3 in a manner that the hot compressed refrigerant gas discharged from the compressor 1 is cooled via heat exchange with a flow of ambient air which passes over the cooling surfaces of the condensor. A liquid tank 4 is arranged downstream of the condensor 3 and adapted to separate and temporarily store any water and/or foreign matter from the liquified refrigerant before it is fed into an evaporator 5 via a temperature responsive expansion valve 6. This valve, as shown, is provided with a temperature sensor arrangement in the form of a bulb 7 filled with volatile fluid which is disposed in contact with the discharge port of the evaporator 5 so as to be exposed to the temperature of the gas which is discharged therefrom. The discharge port of the evaporator is, as shown, directly connected to the induction port of the compressor.
FIG. 2 shows a prior art type wobble plate compressor which has been used in the above described refrigeration system. A device of this nature is disclosed in U.S. Pat. No. 4,428,718 issued on Jan. 31, 1984 in the name of Skinner.
This device is arranged to be responsive to the pressure of the gaseous coolant which is fed to the compressor so that when the thermal load on the system is low such as tends to occur when the ambient temperature is low, the displacement of the compressor is reduced to avoid wasteful energy consumption. On the other hand, when the thermal load on the system is high, the displacement of the compressor is increased to accomodate the increased heat removal requirements.
In more detail, this arrangement includes a cylinder block 100 having a cylinder head 102 and a crankcase cover 104 sealingly secured to opposite ends thereof. A drive shaft 106 is supported centrally in the compressor at the cylinder block 100 and the crankcase cover 104 by radial needle bearings 108, 109 respectively. The drive shaft 106 is further axially retained by a thrust washer 110 which is disposed inward of the radial bearing 108 and a thrust needle bearing 112 located inboard of the radial bearing 109. The drive shaft 106 extends through the crankcase cover 104 for connection to prime mover such as an automotive engine or the like (not shown) by way of a pulley 114 which is selectively connectable to the drive shaft via an electromagnetic clutch 116.
The cylinder block 100 has five axially arranged parallel cylinders bores 120 (only one shown) which are equally spaced from the axis of the drive shaft 106 and arranged equidistantly thereabout. In this arrangement the cylinder bores 120 each receive a reciprocal piston 122 which includes seals (no numerals) which are received in annular grooves formed about the periphery thereof. Each of the pistons 122 is connected to an annular nonrotary wobble plate 124 via a piston rod 125. Each of the piston rods 125 is connected to its respective piston 122 by way of a universal joint. In this arrangement the joints are defined by a spherical rod end which is retained in a socket formed in the backside of the piston by a retainer which is swagged in place. The other end of each of the piston rods 126 is connected to the wobble plate 124 via a similarly constructed universal joint.
The wobble plate 124 is disposed coaxially about the drive shaft 106 and mounted on a rotary drive plate 126. The wobble plate 124 is supported on this drive plate 126 by a thrust bearing 128 and a radial bearing 130 and retained in place by a thrust washer 131 and a snap ring 132.
The drive plate 126 is operatively connected to the drive shaft 106 via a lug 134 which extends from the drive shaft 106 and protrudes through an axial slot (no numeral) formed in a sleeve member 135. The connection between the drive plate 126 and the lug 134 is achieved by a pin 136 which is received in a slanted slot 137. This arrangement permits the drive and wobble plates 126, 124 to undergo changes in inclination within the crankcase in a manner which permits the length of the piston strokes to change as will become more apparent hereinlater.
In order to prevent the wobble plate 124 from rotating with the drive plate 126 in the crank case, the lower end thereof (as seen in the drawings) is operatively connected to a guide pin 138 which is press fitted into bores formed in the cylinder block 100 and the crank case cover 104. Connection between the guide pin 138 and the wobble plate 124 is established by a ball guide 140 which is slidably received on the pin 138. The ends of the ball guide 140 are received in semi-cylindrical guides 141 (only one shown) which are disposed in a slotted section of the wobble plate 124. With this arrangement the ball guide 140 can rotate and move radially inwardly and outwardly in accordance with the change in angle of the wobble plate 124 within the crank case.
A valve plate 144 is sandwiched between the cylinder head 102 and cylinder block 100. This plate 144 closes the open ends of the cylinder bores 120 and is formed with a plurality of inlet and outlet ports 146, 148 which are respectively controlled by reed valves 150, 152. The reed valves 150 which control the inlet or induction ports 146 are arranged to be weaker than those which control the discharge or exhaust ports 148.
The cylinder head 102 is formed with an induction chamber 160 and a discharge chamber 162. These chambers respectively provide fluid communication between a main inlet port 164 formed in the cylinder head and all of the inlet valves and a main discharge port 166 (also formed in the cylinder head) and all of the exhaust valves.
A valve arrangement 170 which controls the pressure prevailing in the crank case is disposed in the cylinder head 102. The arrangement includes a chamber 172 in fluid communication with the discharge chamber 162; a ball valve 174 which control communication between the chamber 172 and bores 175, 176 which lead to the crank case; a chamber 178 in fluid communication with the induction chamber 160; a bellows 180 responsive to the induction pressure; a second valve element 182 which is arranged to control communication between the chamber in which the bellows 180 is disposed and bores 183, 184 which lead to the crank case. The arrangement further includes a valve rod 186 which is connected to the second valve element 182 and which lifts the ball valve element 174 (first valve) off its seat when the bellows 180 expands and urges the second valve element 182 to a closed position.
This valve arrangement 172 functions to control the angle of the drive and wobble plates 126, 124 within the crank case and therefore the length of the piston strokes. Viz., when the thermal load on the air conditioning system is high, the temperature and pressure of the gaseous refrigerant which is inducted into the induction chamber 160 are high. In response to this the bellows 180 is arranged to contract and induce the second valve 182 to open and the first 174 to close. This tends to induce the situation wherein the pressure in the crank case tends to become equal to that prevailing in the induction chamber 160.
Under these conditions, the pressure differential acting across the pistons 122 is such that a moment of force which tends to turn the wobble plate 124 toward its maximum amount of inclination within the crank case, is produced. This of course tends to lengthen the stroke of the piston towards its maximum valve increasing the capacity of the compressor.
Conversely, if the load on the air conditioning system is low, the relatively high discharge pressure is supplied into the crank case by the reverse setting of the first and second valves. Under these conditions the pressure in each cylinder undergoing induction is lower than that prevailing in the crank case. Thus, the pressure differential acting across the respective piston produces a moment of force via which the wobble plate 124 tends to be tilted toward its minimum amount of inclination. Subsequently, as the compression stroke advances the pressure in the cylinder becomes higher than that prevailing in the crank case whereby the moment of force which tends to turn the wobble plate 124 back toward its maximum inclination is produced. The equilibrium which is established between the two moments determines the actual angle of inclination and the stroke of the piston.
In the event that the load on the compressor drops below that which can be compensated by a change in the inclination of the wobble plate 124 the electromagnetic clutch 116 can be conditioned to disconnect the drive shaft from the prime mover.
For further details relating to the operation of this device reference may be had to the United States Patent referred to hereinbefore, the content of which is hereby incorporated by reference thereto.
However, with the above disclosed type of compressor a drawback has been encountered in that as the wobble plate 124 is held stationary and the drive plate 126 rotated relative thereto, a small clearance therebetween is inevitably required. However, this clearance which tends to increase with the passing of time, permits a small amount of movement or play between the two members which lead to the generation of noise when the drive plate is induced to rotate at high speeds. This tends to reduce the working life of device as well as being noisy.